Inverters are circuits for converting DC power to AC power. One type of inverter circuit uses a parallel resonant circuit as the output stage.
A parallel resonant circuit has an inductor in parallel with a capacitor. The inductance of the inductor and the capacitance of the capacitor determine the resonant frequency of the parallel resonant circuit.
The inverter operates at an inverter frequency. Usually, the inverter frequency is at slightly higher frequency than the resonant frequency of the parallel resonant circuit.
Parallel resonant inverter circuits produce at the output a constant voltage independent of load. Thus, with fluorescent lamps, it is possible to easily connect the lamps in parallel. This is advantageous because removal of one lamp will not result in the extinguishment of all other lamps.
Parallel resonant circuits may be relatively expensive. The parallel resonant inverter circuits must be "current fed", i.e., a source of constant DC current must be provided to the parallel resonant inverter circuit. Providing a source of constant DC current requires a very large inductor. A large inductor requires an inductor with a large number of turns, thus making the inductor costly.
Further, when the load coupled to the parallel resonant circuit, the power produced by the parallel resonant circuit is not decreased. For example, if a parallel resonant inverter energizes a number of lamps, if one lamp is removed, increased power is supplied to the remaining lamps, causing the lamps to noticeably brighten. In such a situation, the life of the lamp may be reduced by the changes in the power supplied to the lamps.